NbTi has long served as the backbone of the superconducting wire industry. Despite continued research into alternate materials, including the recent surge of interest in high-T.sub.c superconductors, NbTi remains the superconductor of choice. Unfortunately, high quality NbTi wire is difficult to produce, requiring a complicated and time-consuming schedule of heat treatments. A composite material with the promise for rendering such heat treatment schedules unnecessary is desirable.
While a number of heat treatment schedules can be and are currently employed in the production of NbTi conductors, their purpose is the same: to precipitate .alpha.-Ti particles in the NbTi. These particles serve as defects capable of pinning flux lines. This pinning in turn allows high J.sub.c 's to be achieved. The drawbacks to the heat treatment method of processing are numerous, but may be briefly summarized as follows:
1. The balance between the number and length of heat treatments, the pre-heat treatment strains and heat treatment temperatures is delicate. Hence, optimizing the J.sub.c is difficult, and is vulnerable to errors during the processing of the wire.
2. The need for complicated heat treatment schedules lengthens processing time substantially and increases production costs.
3. The presence of .alpha.-Ti decreases the ductility of NbTi; this is manifested in difficulties in obtaining desired conductor piece lengths.
4. Extensive heat treatment introduces Ti-Cu compound formation problems that can only be avoided through the use of barrier materials.
It is well-known that .alpha.-Ti precipitates in NbTi greatly enhance J.sub.c due to their ability to serve as flux pinning sites. These precipitates, which form at .beta.-Nb-Ti grain boundaries, are generally created by a series of heat treatments separated by strain resulting from cold deformation. This strain encourages the .alpha.-Ti precipitation. A final, larger strain occurs after the last heat treatment. The final strain elongates the .alpha.-Ti and allows optimization of the J.sub.c.
The particulars of the heat treatment schedule depend upon a number of factors: NbTi composition, homogeneity, etc. However, a typical schedule for the commonly used Nb46.5 wt % Ti will involve three or more 300.degree. C.-450.degree. C. treatments, 40-80 hours in duration separated by areal reductions of approximately 1.6. The final areal reduction is usually in the range of 7-12.
The best of these schedules produces about 20 volume percent of .alpha.-Ti in the NbTi and J.sub.c 's in excess of 3000 A/mm.sup.2 at 5T and 4.2.degree. K. In wires with these properties, the .alpha.-Ti is configured in a dense array of ribbons 10-20 .ANG. in thickness, 40-80 .ANG. apart, and with the aspect ratio dependent upon the final strain imparted (see, for example, P. J. Lee, J. C. McKinnell, and D. C. Larbalestier, "Restricted Novel Heat Treatments for Obtaining High J.sub.c in Nb46.5 wt % Ti", To be published, presented as paper #HX-03 at ICMC/CEC, Los Angeles, Calif., Jul. 25, 1989).
Clearly, the heat treatment method of producing pinning sites is not only time consuming but is also open to potentially disastrous errors. If heat treatment times or temperatures are incorrect, or if too much or too little strain is applied, initially good material can be rendered useless. A material that does not require such complicated processing and which reliably yields high J.sub.c 's would thus be of tremendous value. Artificially pinned NbTi promises to be such a material.
Work performed several years ago by G. L. Dorofejev, E. Yu. Klimenko, and S. V. Frolov, ("Current-Carrying Capacity of Superconductors with Artificial Pinning Centers", Proceedings of the 9.sup.th International Conference on Magnet Technology, MT-9, Swiss Institute of Nuclear Technology, P. 564-6, Zurich, 1985, ISPN 3-907998-00-6,) demonstrated for the first time that transition metals could be utilized as pinning sites in NbTi. These investigators produced wires containing a Nb50 wt % Ti matrix surrounding up to 10.sup.7 microfilaments of Nb, Ti, or V. The microfilament spacings were equal to the microfilament diameters. These composites were processed without heat treatment to a variety of sizes for J.sub.c testing. It was found that J.sub.c increased in inverse proportion to the microfilament diameter down to 500 .ANG.. Below this size, mechanical and diffusional effects began to degrade the properties. The best of the composites, incorporating Nb filaments in the NbTi matrix, displayed a J.sub.c of 3500 A/mm.sup.2 at 5T and 4.2.degree. K.
In work performed by I. Hlasnik et al., ("Properties of Superconducting NbTi Superfine Filament Composites with Diameters &lt;0.1 .mu.m", Cryogenics, vol. 25, October, 1985) Cu-NbTi composites consisting of 9,393,931 NbTi filaments embedded in Cu were fabricated via multiple restacks. No special heat treatments were employed during processing. NbTi filament diameters as low as 200 .ANG. were achieved, along with Cu matrix thicknesses of 100 .ANG.. FIG. 1 shows a plot of critical current density versus filament diameter for a Cu-NbTi composite at 5T and 4.2.degree. K. The peak J.sub.c of approximately 3000 A/mm.sup.2 occurs at 500 .ANG. filament diameter and is followed a rapid decline, consistent with the findings of Dorofejev et al.
Recent work by L. R. Motowidlo, H. C. Kanithi, and B. A. Zeitlin, ("NbTi Superconductors with Artificial Pinning Structures", To be published, presented as paper #HX-01 at ICMC/CEC, Los Angeles, Calif., Jul. 25, 1989, see also U.S. Pat. No. 4,803,310,) transposed the positions of the Nb and NbTi relative to the approach of Dorofejev et al., placing the NbTi within a Nb matrix. Employing multiple restacks, the investigators produced a multifilament wire containing 1250 filaments. Each of these copper clad filaments contained 5800 Nb46.5 wt % Ti cores within a Nb matrix. The Nb and NbTi occupied equal volume fractions within the filaments. By simply drawing the multifilament material down, the investigators achieved J.sub.c 's as high as 3700 A/mm.sup.2 in a 0.020" diameter wire at 4T and 4.2.degree. K., corresponding to Nb and NbTi dimensions of about 150 .ANG. (matrix thickness) and 670 .ANG. (core diameter), respectively. A plot of J.sub.c versus applied field for several of the wire diameters tested is shown in FIG. 2. These data were taken at 4.2.degree. K. using a standard four point probe and helically wound samples. The voltage taps were separated by 75 cm.
The remarkably good low field behavior found by the investigators is mitigated by the poor high field behavior. H.sub.c 2 for the material was found to be only about 7.5T, well below that for conventionally processed material (11T). Nonetheless, the experiment clearly demonstrated transition metal pinning.
Older work by Roberts et al. U.S. Pat. No. 3,625,662, Dec. 7, 1971 and MacDonald, U.S. Pat. No. 4,414,428, Nov. 9, 1983, involved the fabrication of superconductors using layered sheet materials, but in neither case did the investigators contemplate the engineering of pinning mechanisms via the manipulation of superconducting and normal metals.